In a continuous casting steel-making process, molten steel is poured from a ladle into a large holding vessel known as a tundish. The tundish has one or more outlets through which the molten steel flows into one or more respective moulds. The molten steel cools and begins to solidify in the moulds to form continuously cast solid lengths of metal. A submerged entry nozzle is located between the each tundish outlet and each mould, and guides molten steel flowing through it from the tundish to the mould. A stopper rod controls the flow rate of the molten steel through the submerged entry nozzle.
The stopper rod generally comprises an elongate body having a rounded nose at one end thereof. In use, the rod is orientated vertically along its axis and is disposed with its nose adjacent the throat of the submerged entry nozzle such that raising and lowering of the stopper rod opens and closes the inlet of the submerged entry nozzle and thereby controls the flow of metal therethrough. The nose of the stopper rod is sized to completely close the inlet of the submerged entry nozzle when lowered to a seated position within the throat of the submerged entry nozzle.
A particular problem associated with the casting of molten metal is that inclusions (e.g. alumina) are often present in the molten metal as it is flowed from the tundish to the mould. Such inclusions tend to deposit on the stopper rod nose or within the submerged entry nozzle depending upon the flow conditions within the casting channel. Accordingly, over time the build up of inclusions can affect the geometry of the components to such an extent that the flow control characteristics of the system are altered and the continuous casting sequence may have to be interrupted.
The injection of an inert gas, such as argon, down the centre of the stopper rod and out of a discharge port in the nose of the stopper alleviates alumina build up and clogging. However, the venturi effect of molten metal flowing past the stopper in the throat of the nozzle creates a negative pressure which can be transmitted back into the stopper rod through the discharge port, potentially sucking air into the metal through the stopper if any joints are not airtight. To date, this problem has been addressed by providing a restriction at the interface between the body and the nose of the stopper rod. The restriction may be a simple narrowing of the bore or may be constituted by a plug with a narrow bore therethrough (or a porous plug) fixed in the stopper bore. The restriction creates a backpressure and results in a positive internal pressure in the stopper rod upstream of the restriction. This positive internal pressure inhibits air ingress into the argon supply channel thereby reducing the quantity of contaminants in the metal being cast.
It will be understood that all references to pressure are relative to atmospheric pressure so that negative pressures relate to pressures below atmospheric pressure and positive pressures relate to pressures above atmospheric pressure.
A disadvantage of using a typical restriction, such as that described above, is that over time an increase in internal pressure can arise which can result in the stopper rod cracking or even being blown apart.
It is therefore an aim of the present invention to provide a stopper rod that addresses the afore-mentioned problems.